Preheat oven to 180Â°C/160Â°C fan-forced. Heat a large non-stick frying pan over a medium heat. Add oil, butter and onion. Cook until onion is soft and translucent, cool slightly. Combine onion with remaining ingredients in a large bowl and stir well. Transfer mixture to a 1-litre-capacity baking dish and cook in preheated oven 30 minutes or until cooked through and crisp on top.